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This report, which was supported by the Office of Naval Research under contract N7onr-29529 with the Antenna Laboratory, Institute of Engineering Research, deals with rectangular apertures in the wall of a waveguide whose dimensions are such that the length-to-width ratio is of the order of unity, the length being in the neighborhood of a half wavelength. We speak of these elements loosely as slots, though in the strict sense of the word a slot is an aperture whose width is small compared with its length. The properties of the larger aperture elements are not known as well as those of the narrow slot. The study of the properties was undertaken both for its bearing on the general theory of apertures and for the potentialities of aperture elements for antenna design work. Whereas the nature of the excitation of a narrow slot is a function of its length primarily, that of the larger aperture element is strongly dependent on its position on the waveguide wall and its orientation with respect to the dominant mode in the guide. The excitation also depends on the nature of the termination of the waveguide beyond the aperture. The greater variety of inclinations that can thus be obtained suggests possible developments in beam synthesis techniques.